The present invention relates to a novel polyimide copolymer and a metal laminate using the same and more particularly to a novel polyimide copolymer effective for use preferably in bonding with metallic foils and a metal laminate using the same.
Heretofore, metal laminates such as flexible wiring boards, etc. have been prepared by bonding metallic foils such as copper foils, etc. to aromatic polyimide films, using an adhesive of epoxy resin, urethane resin, etc. However, flexible wiring boards prepared by using such an adhesive have suffered from various problems due to the adhesive used, such as peeling of the adhesive layer due to high temperatures in the soldering step or use in high temperature circumstances, smear generation in the drilling step, etc.
To overcome such problems, it would be better to conduct direct lamination of aromatic polyimide and metallic foils without use of the adhesive layer, but the peel strength of the most of the resulting metal laminates has not been satisfied yet.
An object of the present invention is to provide an aromatic polyimide capable of forming a metal laminate by direct lamination with metallic foils and fully satisfy the peel strength of metal laminates thus formed.
Such an object of the present invention can be attained by a novel polyimide, which is a copolymer of isopropylidene-bis-(4-phenyleneoxy-4-phthalic acid)dianhydride and 6-amino-2-(p-aminophenyl)benzimidazole or a copolymer of two kinds of tetracarboxylic acid dianhydrides consisting of isopropylidene-bis-(4-phenyleneoxy-4-phthalic acid)dianhydride and 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid dianhydride and 6-amino2-(p-aminophenyl)benzimidazole.
For the tetracarboxylic acid dianhydrides of the present novel polyimide copolymers, isopropylidene-bis(4-henyleneoxy4-phthalic acid)dianhydride: 
or a mixture thereof with 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid dianhydride: 
can be used
When component (A) and component (B) are used together, component (B) is used m a proportion of not more than about 90 mol. %, preferably not more than about 80 mol. % to tie mixture thereof with component (A). When component (B) is used in a higher proportion, the proportion of component (A) will be correspondingly not more than about 10 mol. %, and the solubility of the resulting polyimide copolymer in an organic solvent will be lowered
For diamine that reacts with these tetracarboxylic acid dianhydrides, 6-amino-2-(p-aminophenyl)benzidazole can be used. 
Reaction of tetracarboxylic acid dianhydride with diamine is carried out even in an aprotic polar solvent such as dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, etc., but preferably in a polar solvent such as m-cresol, etc. Practically, a solution of diamine in a polar solvent is dropwise added to a solution of tetracarboxylic acid dianhydride in a polar solvent with stirring while keeping a temperature at about 0xc2x0-about 60xc2x0 C., and after the dropwise addition reaction is carried out with stirring at a temperature of about 0xc2x0-about 60xc2x0 C. for about 0.5-about 5 hours. It seems that polyamic acid is formed by the reaction. To complete polyimidization reaction by dehydrating cyclization reaction, heating to about 100xc2x0-about 250xc2x0 C., preferably about 150xc2x0-about 200xc2x0 C., is carried out with stirring for about 2-about 8 hours in the latter half stage of reaction. A catalyst such as benzoic acid, etc. is added to the solution of tetracarboxylic acid dianhydride and used for the reaction.
The reaction m is poured into an insoluble organic solvent such as methanol, etc. to obtain white polyimide copolymer The resulting copolymer has a glass transition temperature (Tg) of about 250xc2x0-about 330xc2x0 C. and xcex7red (N-methyl-2-pyrrolidone) of about 0.2-about 3.0 dl. When the copolymer is applied to lamination of metallic foils, the reaction mixture in a solution state can be directly applied to lamination of metallic foils without such separation of polyimide copolymer from the solution of polyimide copolymer as the reaction mixture.
By reaction of tetracarboxylic acid dianhydride (A) with diane (C), a polyimide copolymer with the following repeat units can be obtained: 
When tetracarboxylic acid dianhydride (A) is used together with (B), a polyimide copolymer with the following repeat units in addition to the above-mentioned repeat units can be obtained by reaction with diamine (C): 
Preparation of metal laminate using such a novel polyimide copolymer can be carried out by applying a solution of polyimide copolymer to a metallic foil, typically a copper foil by casting, etc., followed by heating in two stages, ie. a temperature of about 60xc2x0-about 200xc2x0 C., preferably about 80xc2x0-120xc2x0 C., and another temperature of about: 150xc2x0-about 200xc2x0 C., each for about 5-about 60 minutes, thereby forming a polyimide copolymer layer having a film thickness of about 3-about 75 xcexcm, i.e. forming a metal laminate with the metallic foil at one side of the copolymer layer. Another metallic foil is laid on the other side of the copolymer layer, followed by passing through and between laminate rolls heated to a temperature of about 150xc2x0-about 400xc2x0 C., preferably about 200xc2x0-350xc2x0 C. with processing, thereby easily bonding the metallic foil directed specially to the transfer to the copolymer layer, i.e. forming a two-side laminate such as copper-lined material etc.
The present invention will be described below, referring to Examples.